1. Field of the Invention
This invention relates to the purification and controlled release of water by pervaporation, and specifically relates to the use of a hydrophilic membrane in an apparatus that allows the direct use of pure or impure water for irrigation, rehydration or drinking.
2. Description of the Related Art
There are known processes for purifying water, and the type of process used depends on the nature and amount of the impurities in the water. For example, impurities in particulate form and in solution may both need to be removed from the water. The aim is to purify the water so that it contains sufficiently low amounts of suspended particles, suspended microbes and dissolved salts to meet the quality requirements of water for drinking, for the production of food and drink, for agricultural irrigation and for industrial use.
Processes for the purification of water are usually classified as filtration, distillation or reverse osmosis. In conventional particle filtration processes, impurities in particulate form, such as suspended inorganic particles, are removed using porous constructions such as woven or non-woven fabrics. In cases where very small particles must be filtered, polymer membranes are used which are microporous, that is, the membranes have very small holes through which the particulates to be filtered cannot pass.
Aqueous solutions containing dissolved salts are usually purified by reverse osmosis or distillation. When the aqueous solution is in the form of seawater or brackish water, these processes are generally known as desalination. The process of reverse osmosis relies on applying pressure to solutions of ions across a semi-permeable membrane. If the applied pressure is greater than the osmotic pressure of the solution, purified water is collected from the side of the membrane not in contact with the solution. Reverse osmosis membranes let water pass through them but reject the passage of salt ions. In reality a small percentage, say 1%, of sea salts pass through the membranes. U.S. Pat. No. 5,547,586 discloses a method to desalinate seawater and brackish water utilizing an enzyme-assisted membrane. As opposed to reverse osmosis, distillation methods using seawater or brackish water can yield water with a very low amount of suspended particles and dissolved solids. However, the high latent heat of vaporization of water means that distillation processes require a high input of energy and therefore generally operate at a higher cost compared to reverse osmosis processes.
It has been known from U.S. Pat. No. 4,725,481 that a copolyetherester elastomer can be used, on its own or as a part of a bicomponent film of a hydrophobic layer and a hydrophilic layer of copolyetherester elastomers bonded together, to permit the differential transfer of water vapor to prevent buildup of moisture such as in a surgical drape or in waterproof apparel equipment.
The present invention generally relates to a method and an apparatus for the purification and/or the controlled release of water by pervaporation, by passing water vapor contained in air, or liquid water that may contain suspended or dissolved impurities, including but not limited to seawater, brackish water or other kinds of polluted water, through one or more layers of hydrophilic membranes to remove impurities from the water.
The layer or layers of hydrophilic membranes may be present either in the form of an unsupported structure or coated on or adhered to a support material, wherein the hydrophilic membrane layer can be a copolyetherester elastomer, a polyether-block-polyamide, a polyether urethane, homopolymers or copolymers of polyvinyl alcohol, or mixtures thereof.
A preferred hydrophilic membrane layer is made from a hydrophilic polymer having a water vapor transmission rate according to ASTM E96-95 (Procedure BW) of at least 400 g/m2/24 h, measured using air at 23xc2x0 C. and 50% relative humidity at a velocity of 3 m/s on a film of total thickness 25 microns. A more preferred hydrophilic membrane layer is made from a hydrophilic polymer having a water vapor transmission rate according to ASTM E96-95 (Procedure BW) of at least 3500 g/m2/24 h, measured using air at 23xc2x0 C. and 50% relative humidity at a velocity of 3 m/s on a film of total thickness 25 microns.